docs: alabaster theme: better contrast for background color of topics

The old value just was way to heavy, and notes/warnings/...
admonitions did not stick out anymore.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/custom.css

@@ -14,6 +14,10 @@ pre {
     padding: 5px 10px;
 }
 
+div.topic {
+    background-color: #FAFAFA;
+}
+
 li a.current {
     font-weight: bold;
     border-bottom: 1px solid #000;